Xerographic process



y 7, 1966 R. w. GUNDLACH 3,251,686

XEROGRAPHI C PROCES S Filed July 1, 1960 INVENTOR. ROBERT W.. GUNDLACH ATTORNEV United States Patent 3,251,686 XEROGRAPHIC PROCESS Robert W.Gundlach, Spencerport, N.Y., assignor to Xerox Corporation, acorporation of New York Filed July 1, 1960, Ser. No. 40,357 3 Claims.(Cl. 961) The present invention relates gene-rally to Xerographicplate-s and more particularly to a novel method of fabricatin g xerographic plates.

The xerographic process is described in US. 2,297,691 to Chester F.Carlson and involves the sensitization of a Xerographic plate (as byplacing an electrostatic charge thereon) and the exposure of thesensitized plate to an original image to be reproduced. The exposedplate is developed by contacting the plate surface withelectrostatically-charged, finely divided powder particles to produce apowder image which is either used or fixed in situ or thereaftertransferred from the plate to a final support, the transferred imagebeing fixed thereon to form the final print. If desired, the transferstep may be omitted and the image fixed to the plate itself.

As originally described by Carlson, the xerographic plate consisted of athin layer of sulfur, anthracene or anthraquinone, either singly or incombination, applied to a relatively conductive base by melting andflowing onto the base or by evaporating the material onto the base whichis kept at a lower temperature so as to condense the vapor. I

A tremendous advance was made in xerography when it was discovered thatvitreous selenium was highly photoconductive. A selenium xerographicplate generally comprises a metal backing plate, as aluminum, havingcoated on one side, as by vacuum evaporation, a layer of very highpurity vitreous selenium. In the dark the selenium layer has anextremely high resistivity, but when exposed to light the resistivity isreduced many orders of magnitude, the amount depending on the intensityand wavelength of the light. By reason of its high electricalresistivity in the dark the selenium layer can be chargedelectrostatically, which charge is retained for a prolonged periodshould no light impinge thereon. The outstanding ability of vitreousselenium to hold a charge for an appreciable period in the dark coupledwith its high light sensitivity have made the selenium plate thestandard commercial plate of xerography. Such plates are costly tofabricate, but may be used a thousand or more times in the xerographicprocess so that the cost per image developed is small. Thus, theselenium plate requires reusability to obtain reasonable operatingcosts.

Another advance was made in the field of xerographic plates with thediscovery of the binder plate. Such plates are described by Arthur E.Middleton in US. Patent 2,663,636. As there described it was found thatan efficient xerographic plate can be obtained by coating a relativelyconductive base with a photoconductive insulating composition preparedby intimately mixing and grinding together any photoconductiveinsulating material, a binder of high electrical resistance and asolvent.

Whether the xerographic plate comprises a uniform layer ofphotoconductive insulator or a finely ground photoconductor dispersed ina resin binder, it is often desirable to protect the light-sensitivesurface by overcoating the surface with a protective material.Generally, the overcoating is formed by applying a solvent solution ofan organic resin to the plate surface and allowing the solution toevaporate. However, many of the highly polymerized, solvent resistantresins now available can not be applied by this method. Further, it hasbeen found that the solvents necessary often have a deleterious effectupon the photoconductor. Thus, for example, as

now US. Pat. No. 3,121,006.

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described in pending U.S. patent application Serial No. 482,896, filedJanuary 19, 1955, by Harold E. Clark, and now abandoned, where theovercoating is applied to a binder plate, it has been found that thesolvent for the overcoating must be so selected as not to attack thebinder. Where the solvent softens the binder, it has been found tomaterially affect the image forming qualities of the xerographic plate.In the case of the silicone resin now widely used commercially in thepreparation of binder plates, the resin binder dissolves in such a widevariety of solvents that the selection of the proper solvent for theovercoating is exceedingly diificult. In the case of selenium, it hasbeen found that many solvents induce crystallization of the seleniumcausing it to convert to metallic selenium. This allotropic form ofselenium, that is, crystalline selenium, has too low a resistivity tosupport an electrostatic charge.

I have now found a process whereby highly polymerized, solvent resistantresin overcoatings may be applied to a xerographic plate. There is thusobtained a highly flexible reusable plate characterized by exceptionalease of cleaning and overall efficiency in a repetitive xerographicprocess. The photoconductor may be either in the form of a continuousuniform layer as in the case of vitreous selenium or may be in the formof a binder plate. Preferred binder-photoconductor combinations aredescribed in the co-pending application of Middleton and Reynolds, Ser.No. 668,165, filed June 26, 1957, The process involves reversing thenormal method of preparation of the xerographic plate, that is, ratherthan applying the photoconductive material to an electrically conductivesupport member, the photoconductive material is instead applied to athin pellicle of a highly polymerized, solvent resistant resin. A thinelectrically conductive film is then applied on the free surface of thephotoconductor. Particularly preferred overcoatings suitable for use asdescribed are polytetrafiuoroethylene and polyester resins particularlypolyethylene terephthlate. In addition, other materials which may beused as described are polyamide films such. as those prepared fromcaprolactam and nylon 66. Suitable films may also be prepared from poly-.acrylonitrile and copolymers thereof as is well known to those skilledin the art. The resin film can be no more than about 0.5 mil thick andpreferably is no more than about 0.25 mil thick. The conductive coatingon the free surface of the photoconductor may be formed by vacuumevaporating a thin film of metal, as aluminum, thereon, or, if thephotoconductive film is highly temperature sensitive, by vacuumevaporating copper iodide which is electrically conductive and has avery low temperature for evaporation. Alternatively a colloidaldispersion of graphite may also be sprayed or painted on the freesurface to provide the electrically conductive layer. If desired theconductive layer need not be an integral part of the plate structure inwhich case the free photoconductive insulating surface is contacted withan electrically conductive support member during the necessary steps ofthe xerographic process.

In the drawing the figure represents a xerographic plate according tothe instant invention. As shown the figure illustrates a xerographicplate 10, comprising a pellicle of translucent, electrically insulatingsolvent-resistant, highly polymerized resin 13, having coated thereon aphotoconductive insulating layer 12 covered by a thin electricallyconductive layer 11. In order to more clearly depict the structure ofthe plate, the drawing is not to scale. However, as is clearly shown,the conductive layer 11 is not a support layer but rather is merely athin flexible layer providing the necessary electrical ground plane toone side of the photoconductive insulating layer 12. In the instantstructure the support layer is the insulating pellicle 13 which, beingitself flexible, results in a flexible reusable plate 10. Any of thephotoconductive insulating layers known to those skilled in the art maybe used in formulating plates according to the instant invention. Suchphotoconductive insulating layers are described as to preparation,composition, thickness and other parameters, for example, in US.2,803,542 to Ullrich; 2,803,541 to Paris; 2,745,327 to Mengali;2,863,768 to Schaffert; US. application S.N. 526,781 .filed August 5,1955, by Bixby now U.S. Patent No. 2,970,906; and the aforesaidapplication of Middleton and Reynolds.

The general scope and nature of the invention having been set forth, thefollowing examples are given as typical illustrations of the inventionand not by way of limitation.

Example 1 A quarter mil film of polyethylene terephthlate (obtained fromE. I. du Pont de Nemours & Co. under the trade name Mylar) was attachedto a platen and placed in a bell jar. The air was then evacuated fromthe system and a layer of selenium about 20 microns thick evaporatedonto the Mylar. a crucible containing aluminum substituted for thecrucible from which the selenium had been evaporated. A layer ofaluminum a few microns thick was then deposited on the selenium. Theplate so prepared was then tested in the xerographic process utilizing acommercial charging and exposure apparatus obtained from Haloid XeroxInc., Rochester, New York, under the trade name Xerox Copier Model D.The resulting electrostatic image formed on the selenium was madevisible by cascading over the Mylar surface a mixture-of toner andcarrier as described in US 2,618,552. The development apparatus usedtherefor was the commercial developing tray forming a part of the ModelD. A succession of images were developed in this manner, the previousimage being cleaned by rubbing with a linen cloth, with absorbentcotton, with paper tissue and by wiping with the hand. All of thecleaning processes were completely operable for removing the previoustoner image. The toner image could also be easily transferred to asuitable support material such as metal, paper or plastic.

Example 2 A xerographic plate was prepared by applying to a film ofquarter mil Mylar a coating mixture comprising 2.5 parts, by weight, ofzinc oxide to 1 part of a silicone resin obtained from The GeneralElcetric Company under the trade name SR-82, the resin-pigment mixturebeing dispersed in toluene. The resulting binder film was about 35microns thick. The free surface of the photoconductive layer was thenswabbed With a graphite dag obtained from Acheson Colloids Inc., PortHuron, Michigan. The resulting xerographic plate was then utilized inthe xerographic process and cleaned as described in Example 1. Againeasy cleanability and high resistance to scratching was obtained.Heretofore, thebacking member of the Xerographic plate, that is theelectrically conductive or semiconductive layer, has been utilized asthe support member for the Xerographic plate. One reason for this isthat to prevent excessive loss in image resolution, overcoating layersare generally no more than about 2 microns thick. In the instantinvention it has been found, however, that with plates prepared asdescribed herein the overcoating layerrnay be from about 6 to 12 micronsthick without undue loss of image resolution. It is believed that thereason for the improved properties of the plates prepared herein is thaton application of the resin solution having the pigment particlesdispersed therein unto the overcoating layer, the pigment particles arenaturally oriented relative to the film. Further, gravity aids insettling the pigment particles while the resin solution dries.Primarily, however, it is believed that the polar The vacuum was thenbroken and forces between the pigment particles and the interface withthe pellicle attract the pigment preferentially toward the pellicle. Asa result, the distribution of pigment in the resin in the instantinvention is the reverse of that obtained in the normal process formanufacturing xerographic binder vplates. In particular, such a plate ischaracterized by a relatively high and very uniform concentration ofpigment particles distributed along the light sensitive surface. As aresult of the uniformity and high concentration, enhanced resolution isobtainable.

Further, by eliminating geometrical irregularities, previouslycharacteristic of xerographic binder plates, the localized fringingelectrostatic fields formed in background areas by such irregularitiesare eliminated thereby reducing background deposition of toner particlesduring image development. In addition, plates prepared according to theinstant invention are not only easily cleaned and, hence, highlyreusable, but are also flexible permitting high adaptability to avariety of machine applications. Again, the highly insulating externalsurface permits transfer of toner images to electrically conductivesupport members without danger of arcing through the photoconductivesurface.

With regard to selenium plates prepared in this manner, the resultingplates are highly flexible despite the vitreous (i.e., glass-like)nature of the selenium permitting the use of selenium plates in machineconfigurations requiring such plates.

While the present invention has been described herein as carried out inspecific embodiments thereof, there is no desire to be limited thereby,but it is intended to cover the invention broadly within the spirit andscope of the appended claims. As one such variation, it has been foundthat xerographic toner images may be transferred from a sheet ofpolyetetrafluoroethylene (obtained from E. I. du Pont de Nemours & Co.under the trade name Tefion) to almost any surface with very highefficiency. Since the Teflon has very high resistance to elevatedtemperatures, the Teflon can be heated above the melting point of thetoner and pressed against a sheet of paper whereupon the softened ofliquefied toner is completely transferred to the paper. This transfertechnique will also work with the other overcoating surfaces formed inthe instant invention. This process is particularly useful when theovercoating pellicle is applied to a binder plate as temperatures in therange sufiicient to melt the toner image, rather than harming a bindermaterial, appear to have the beneficial effect of eliminating fatigue.Such a process makes it possible to effect complete image transfer andimage fixing simultaneously. Thus, plate cleaning is unnecessary in sucha process. Because of the chemical resistance of the pellicles used inthe instant process, it is also possible to soften the toner image byvapor or other chemical means instead of by heat and, accordingly, touse the process with photoconductive layers such as selenium which aresensitive to heat.

I claim:

1. A process .of xerography for forming electrostatic images on aflexible xerographic member comprising applying a photoconductiveinsulating material to a pellicle of solvent resistant, highlypolymerized electrically insulating organic resin translucent toactivating radiation, said pellicle being no more than about 12 1,thick, and applying a thin conductive coating to the free surface ofsaid photoconductive insulating material to form a firmly bonded,unitary structure consisting of said pellicle, said photoconductiveinsulating material and said conductive coating, applying a ground tothe side of photoconductive layer not in contact with said pelliclewhile placing sensitizing electrostatic charges on said pellicle, andexposing said photoconductive layer to activating radiation through saidpellicle to thereby form'an electrostatic image on said unitarystructure.

2. A process according to claim 1 whereinsaid photo; conductiveinsulating layer is vitreous selenium.

3. A process according to claim 1 wherein said photoconductiveinsulating layer comprises an insulating organic resin binder havingdispersed therein finely divided particles of a photoconductiveinsulating pigment.

References Cited by the Examiner UNITED STATES PATENTS Carlson 961Butterfield 96-1 Sheldon 961 Ullrich 96-1 Walkup 961 X Schlosser 96--1 XHartmann 96-1 Deubner 96-1 Griggs et a1. 961 Owens 96-1 Dessauer et a1.96-1 Schofi'ert 961 Steinhilper 961 Kostelec et a1 961 Crumley et a1.961 Van Dorn 96-l Jones 96--1 Middleton et a1 96--1 Kins'ella 9 6-1Australia.

OTHER REFERENCES Young et al., R.C.A. Review, December 1954, vol. 15,No. 4, pp. 469-484.

15 NORMAN G. TORCHIN, Primary Examiner.

PHILIP E. MANGAN, Examiner.

I. E. ALI X, R. L. STONE, C. VAN HORN,

Assistant Examiners.

1. A PROCESS OF XEROGRAPHY FOR FORMING ELECTROSTATIC IMAGES ON AFLEXIBLE XEROGRAPHIC MEMBER COMPRISING APPLYING A PHOTOCONDUCTIVEINSULATING MATERIAL TO A PELLICLE OF SOLVENT RESISTANT, HIGHLYPOLYMERIZED ELECTRICALLY INSULATING ORGANIC RESIN TRANSLUCENT TOACTIVATING RADIATION, SAID PELLICLE BEING NO MORE THAN ABOUT 12U THICK,AND APPLYING A THIN CONDUCTIVE COATING TO THE FREE SURFACE OF SAIDPHOTOCONDUCTIVE INSULATING MATERIAL TO FORM A FIRMLY BONDED, UNITARYSTRUCTURE CONSISTING OF SAID PELLICLE, SAID PHOTOCONDUCTIVE INSULATINGMATERIAL AND SAID CONDUCTIVE COATING, APPLYING A GROUND TO THE SIDE OFPHOTO-